The present invention relates to titanium alloys usable at high temperatures, particularly those of the TiAl gamma phase type.
Titanium alloys have found wide use in gas turbines in recent years because of their combination of high strength and low density, but generally, their use has been limited to below 600.degree. C., due to inadequate strength and oxidation properties. At higher temperatures, relatively dense iron, nickel, and cobalt base super-alloys have been used. However, lightweight alloys are still most desirable, as they inherently reduce stresses when used in rotating components.
Considerable work has been performed since the 1950's on lightweight titanium alloys for higher temperature use. To be useful at higher temperature, titanium alloys need the proper combination of properties. In this combination are properties such as high ductility, tensile strength, fracture toughness, elastic modulus, resistance to creep, fatigue and oxidation, and low density. Unless the material has the proper combination, it will not perform satisfactorily, and thereby be use-limited. Furthermore, the alloys must be metallurgically stable in use and be amenable to fabrication, as by casting and forging. Basically, useful high temperature titanium alloys must at least outperform those metals they are to replace in some respect, and equal them in all other respects. This criterion imposes many restraints and alloy improvements of the prior art once thought to be useful are, on closer examination, found not to be so. Typical nickel base alloys which might be replaced by a titanium alloy are INCO 718 or IN100.
Heretofore, a favored combination of elements with potential for higher temperature use has been titanium with aluminum, in particular alloys derived from the intermetallic compounds or ordered alloys Ti.sub.3 Al (alpha-2) and TiAl (gamma). Laboratory work in the 1950's indicated these titanium aluminide alloys had the potential for high temperature use to about 1000.degree. C. But subsequent engineering experience with such alloys was that, while they had the requisite high temperature strength, they had little or no ductility at room and moderate temperatures, i.e., from 20.degree. to 550.degree. C. Materials which are too brittle cannot be readily fabricated, nor can they withstand infrequent but inevitable minor service damage without cracking and subsequent failure. They are not useful engineering materials to replace other base alloys.
Those skilled in the art recognize that there is a substantial difference between the two ordered titanium-aluminum intermetallic compounds. Alloying and transformational behavior of Ti.sub.3 Al resemble those of titanium as they have very similar hexagonal crystal structures. However, the compound TiAl has a face-centered tetragonal arrangement of atoms and thus rather different alloying characteristics. Such a distinction is often not recognized in the earlier literature. Therefore, the discussion hereafter is largely restricted to that pertinent to the invention, which is within the TiAl gamma phase realm, i.e., about 50 Ti-50 Al atomically and about 65 Ti-35 Al by weight.
Room temperature tensile ductility as high as 4% has been achieved in two-phase gamma alloys based on Ti-48Al such as Ti-48Al-(1-3)X, where X is Cr, V or Mn. This improved ductility was possible when the material was processed to have a duplex microstructure consisting of small equiaxed gamma grains and lameliar colonies/grains. Under this microstructural condition, however, other important properties including low temperature fracture toughness and elevated temperature, i.e., greater than 700.degree. C., creep resistance are unacceptably low. Research has revealed that an all-lamellar structure dramatically improves toughness and creep resistance. Unfortunately, however, these improvements are accompanied by substantial reductions in ductility and strength. Recent experiments have shown that the improved fracture toughness and creep resistance are directly related to the features of lameliar structure, but that the large gamma grain size characteristic of fully-lamellar gamma alloys is responsible for the lowered tensile properties. These experiments have also demonstrated that the normally large grain size in fully-lamellar microstructure can be refined.
Kim et al, U.S. Pat. No. 5,226,985, issued Jul. 13, 1993, describe two methods for refining the microstructure of gamma titanium aluminide alloys. The first method is referred to as a thermomechanical process (TMP) and comprises shaping the article by extrusion or hot die forging, rolling or swaging, followed by a stabilization aging treatment. Where shaping is by extrusion, extrusion is carried out at a temperature in the approximate range of 0.degree. to 20.degree. C. below the alpha-transus temperature of the alloy. The alpha-transus temperature (T.sub..alpha.) generally ranges from about 1300.degree. to about 1400.degree. C., depending on the alloy composition. T.sub..alpha. decreases with decreasing Al. The transus temperature has also been shown to decrease with many interstitial (e.g., O and C) and substitutional (e.g., Cr, Mn, Ta and W) alloying elements. T.sub..alpha. can be determined relatively routinely by standard isothermal heat treatments and metallography, or by Differential Thermal Analysis (DTA), provided the material is homogeneous.
The aging temperature can range between 750.degree. and 1100.degree. C., depending on the specific use temperature contemplated, for at least one hour and up to 300 hours. Where shaping is by hot die forging, rolling or swaging, such shaping is carried out at a temperature in the approximate range of 50.degree. C. above T.sub..epsilon., the eutectoid temperature of two-phase gamma alloys (.apprxeq.1130.degree. C.), to about 0.degree. to 20.degree. C. below T.sub..alpha., at a reduction of at least 50% and a rate of about 5-20 mm/min.
The second method is referred to as a thermomechanical treatment (TMT), which comprises hot working at temperatures well below the alpha-transus (T.sub..alpha.) with subsequent heat treatment near the alpha-transus, followed by a stabilization aging treatment. Where shaping is by extrusion, extrusion is carried out at a temperature in the approximate range of T.sub..epsilon. -130.degree. C. to T.sub..alpha. -20.degree. C. Where shaping is by hot die forging, rolling or swaging, such shaping is carried out at a temperature in the approximate range of T.sub..epsilon. -130.degree. C. to T.sub..alpha. 20.degree. C., at a reduction of at least 50% and a rate of about 5-20 mm/min. Where shaping is by isothermal forging, such shaping is carried out at a temperature in the approximate range of T.sub..epsilon. -130.degree. C. to T.sub..epsilon. +100.degree. C., at a reduction of at least 60% and a rate of about 2-7 mm/min. After hot working, the article is heat treated at a temperature in the approximate range of T.sub..alpha. -5.degree. C. to T.sub..alpha. +20.degree. C. for about 15 to 120 minutes. Following such heat treatment, the article is cooled and given an aging treatment.
Kim et al is a valuable contribution to the art. The TMP method disclosed therein provides a product with a fine lameliar microstructure. The TMT method provides a product having a fine, randomly oriented lameliar microstructure.
We have discovered that the processing window can be extended, thus allowing for more realistic and reliable foundry practice. This newly discovered hot working temperature range will generate unique lameliar microstructures consisting of randomly oriented lameliar colonies, with serrated grain boundaries. Gamma titanium aluminide alloys with this structure have the requisite balance of properties for moderate and high temperature aerospace applications: high specific strength, stiffness, fracture resistance and creep resistance in the temperature range of room temperature to about 950.degree. C.
Accordingly, it is an object of the present invention to provide improved methods for producing articles of gamma titanium aluminide alloys.
Other objects and advantages of the invention will be apparent to those skilled in the art.